The proposed research will characterize the organization of two thalamic nuclei in snakes and turtles, providing the first detailed account of thalamic organization in a non-mammalian vertebrate. It will focus on nucleus rotundus and the dorsal lateral geniculate nucleus (DLGN), both of which convey visual information to the telencephalon. The topography of retinotectal and retinothalamic projections will be determined using Fink-Heimer methods following restricted electrolytic lesions of the retina. The topography of tectothalamic projections to DLGN and to nucleus rotundus will be determined following restricted lesions of the tectum and injections of tritiated amino acids. The precise location of retinal and tectal afferents to DLGN will be determined in combined lesional and orthograde transport experiments. The cellular and synaptic organization of DLGN and nucleus rotundus will be determined using Golgi and electron microscopic techniques.